A FLUID INCLUSION STUDY OF NYF-TYPE GRANITIC PEGMATITES

Although there is no single feature that is diagnostic to all pegmatites, they are often mineralogically complex and generally characterized as extremely coarse grained. Most previous work done on pegmatites focuses on their mineralogy and bulk rock chemistry, whereas this study will focus on the geochemical evolution of the precipitating fluids. This project will investigate the fluids involved with NYF-type pegmatites from the MacDonald and West Lake quarries from the greater Bancroft area, Ontario, Canada. This study will present and discuss the geochemical data and petrogenesis of fluids associated with these pegmatites. This will be accomplished by examining fluid inclusions trapped during crystallization of the pegmatites. Fluid inclusions in pegmatites record the fluid composition and evolution of late magmatic fluids in peraluminous granites and associated pegmatites. These trapped fluids would thus represent the exact conditions present during crystallization. Fluid inclusions typically display the complex chemistry of brine solutions that is not necessarily recorded by any one mineral, and in the case of pegmatites rarely considered in single mineral geochemical studies. The salinity of these fluids is often intrinsically related to the depth of crystallization, but can also be reflected by the degree of crystallization and partitioning behavior in the silicate melt. Primary fluid inclusions are trapped at the time of mineral growth while secondary inclusions are trapped along healed fractures. As the primary goal of this study is understanding the precipitating pegmatite fluids, heating and freezing experiments were conducted on primary fluid inclusions defining fluid inclusion assemblages using a Fluid Inc. stage at Stockton College. Multiple generations of polyphase fluid inclusions were found within various mineral phases from each locality. Homogenization and ice-melting temperatures were measured to determine the minimum trapping temperatures and provide an estimate of the fluid salinity, respectively. Finally, the geochemistry of both the inclusions and their host crystals will be determined using laser ablation-inductively coupled plasma mass spectrometry that will allow for in-situ analysis of major and minor elements.